Sensing Device

Abstract

A sensing device includes a semiconductor structure, a substrate, a first electrode and a second electrode, and a heater. A sensing area arranged on the top side of the semiconductor structure. The substrate is located under the bottom side of the semiconductor. The first electrode and the second electrode are arranged on the top side of the semiconductor structure. The heater is disposed on the semiconductor structure and separated from the sensing area by a distance less than 100 .mu.m.

Description

RELATED APPLICATION

[0001] This application claims the benefit of US Provisional Application Ser. No. 62/497,108, filed on Nov. 7, 2016, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

[0002] The present disclosure relates to a sensing device and methods of making the same, and in particular to a sensing device having an embedded heater.

DESCRIPTION OF THE RELATED ART

[0003] The development of a sensing device capable of obtaining precise information within a short reacting time is required for the booming IoT (Internet of Things) market. Particularly, efforts to achieve high precision (low detection limit), low current consumption (good power efficiency), low cost of a sensing device has been continued to implement comfortableness of living spaces, cope with a bad industrial environment, and manage food manufacturing processes, etc.

[0004] A method widely used in sensing device for improving the detection limit and shorten the reacting time is bonded an additional heater to heat up the sensing device. Hence, the sensing device reacts rapidly and accurately measures the concentration of the substances while operating in high temperature. In addition, the substances absorbed on the sensing devices are removed by heating at the high temperature to recover the sensing device. Therefore, the temperature characteristic of the sensing device directly affects the detection limit, the reacting time, the recovery time, and the like of the sensing device. However, the heater providing the effective heating causes a high current consumption and is not suitable in some application.

[0005] It is an object of the current disclosure to provide a sensing device with an embedded heater to improve the detection limit of the sensing device, and shorten the reacting and recovery time under low current consumption operation.

SUMMARY OF THE DISCLOSURE

[0006] The following description illustrates embodiments and together with drawings to provide a further understanding of the disclosure described above.

[0007] A sensing device includes a semiconductor structure, a substrate, a first electrode and a second electrode, and a heater. A sensing area arranged on the top side of the semiconductor structure. The substrate is located under the bottom side of the semiconductor. The first electrode and the second electrode are arranged on the top side of the semiconductor structure. The heater is disposed on the semiconductor structure and separated from the sensing area by a distance less than 100 .mu.m.

[0008] A sensing device includes a semiconductor structure, a substrate, an electrode, and a heater. A sensing area arranged on the top side of the semiconductor structure. The substrate is located under the bottom side of the semiconductor. The electrode is disposed on the top side of the semiconductor structure and exposes the sensing area. The heater is disposed on the semiconductor structure. The sensing device has an operating current less than 350 mA.

[0009] A sensing device includes a semiconductor structure, a substrate, an electrode, and a heater. The substrate is located under the bottom side of the semiconductor. The electrode is disposed on the top side of the semiconductor structure and exposes the sensing area. The heater is disposed on the semiconductor structure. The sensing device has a detection limit less than 10 ppm.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1A shows a top view of a sensing device in accordance with an embodiment of the present disclosure.

[0011] FIG. 1B shows a bottom view of a sensing device shown in FIG. 1A.

[0012] FIG. 1C shows a cross-sectional view taken along line A-A' of a sensing device shown in FIG. 1A.

[0013] FIG. 1D shows a bottom view of a sensing device in accordance with an embodiment of the present disclosure.

[0014] FIG. 2A.about.2E show bottom views of the heaters in accordance with embodiments of the present disclosure.

[0015] FIG. 3 shows a cross-sectional view of a sensing device in accordance with another embodiment of the present disclosure.

[0016] FIG. 4 shows a cross-sectional view of a sensing device in accordance with another embodiment of the present disclosure.

[0017] FIG. 5A shows a top view of a sensing device in accordance with another embodiment of the present disclosure.

[0018] FIG. 5B shows a cross-sectional view taken along line B-B' of a sensing device shown in FIG. 5A.

[0019] FIGS. 6A.about.6F show steps of manufacturing a sensing device in accordance with an embodiment of the present disclosure.

[0020] FIG. 7A shows a top view of a sensing device in accordance with an embodiment of the present disclosure.

[0021] FIG. 7B shows a cross-sectional view taken along line C-C' of a sensing device shown in FIG. 7A.

[0022] FIG. 8A shows a top view of a sensing device in accordance with an embodiment of the present disclosure.

[0023] FIG. 8B shows a cross-sectional view taken along line D-D' of a sensing device shown in FIG. 8A.

[0024] FIG. 9 shows a cross-sectional view of a sensing device in accordance with another embodiment of the present disclosure.

[0025] FIG. 10 shows a cross-sectional view of a sensing device in accordance with another embodiment of the present disclosure.

[0026] FIG. 11 shows a top view of a sensing device in accordance with another embodiment of the present disclosure.

[0027] FIGS. 12A.about.12D show steps of manufacturing a sensing device in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] The drawings illustrate the embodiments of the application and, together with the description, serve to illustrate the principles of the application. The same name or the same reference number given or appeared in different paragraphs or figures along the specification should has the same or equivalent meanings while it is once defined anywhere of the disclosure. The thickness or the shape of an element in the specification can be expanded or narrowed.

[0029] FIGS. 1A.about.FIG. 1C show sensing devices 100 in accordance with embodiments of the present disclosure. FIG. 1A shows a top view of the sensing device 100. FIG. 1B shows a bottom view of the sensing device 100. FIG. 1C shows a partial cross-sectional view taken along line A-A' in FIG. 1A. The sensing device 100 is a field effect transistor (FET) which includes two electrodes 31, 32 (for example, one is source electrode and another is drain electrode), a functionalization layer 9, and a sensing area 4 (not shown in FIG. 1A, underneath the functionalization layer 9) disposed between two electrodes 31, 32. If the sensing device 100 is exposed to an environmental medium (for example, air, exhaust gas) which includes the substance(s) to be detected (target substance), the target substance can react with the sensing area 4. In detail, the target substance can accumulate on the sensing area 4, and the amount of the net surface charge on the sensing area 4 and the electric field between the sensing area 4 and the electrodes 31, 32 are therefore changed. Hence, the sensing device 100 is operated in field effect transistor (FET) mode, and the current (drain-source current) between the electrodes 31, 32 is varied with the concentration of the target substance. The field effect transistor (FET) of sensing device 100 may be selected from the group consisting of a metal-oxide-semiconductor FET (MOSFET), a metal-semiconductor FETs (MESFETs), and a high electron mobility transistor (HEMT).

[0030] The target substance is H.sub.2, NH.sub.3, CO, SO.sub.N, NO, NO.sub.2, CO.sub.2, CH.sub.4, acetone, ethanol, formaldehyde, benzene such as toluene, etc. The target substance can exist in the form of gas or liquid.

[0031] In one embodiment, the sensing device 100 is a HEMT. HEMT structures can be used in a microwave power amplifier as well as a gas or liquid sensing device because of their high two-dimensional electron gas (2DEG) mobility and saturation velocity. As shown in FIG. 1C, the sensing device 100 includes a substrate 1, a hetero-junction structure 2 formed on the substrate 1, a source electrode 31, and a drain electrode 32 located on the top side of hetero-junction structure 2, a sensing area 4 also located on the top side of the hetero-junction structure 2, and a heater 5 located on the backside of the hetero-junction structure 2. The hetero-junction structure 2 includes a first semiconductor layer 21 and a second semiconductor layer 22, and a 2DEG channel 23. The first semiconductor layer 21 and the second semiconductor layer 22 are made of piezoelectric materials and have different bandgaps. The 2DEG channel 23 can be formed at/around the interface of the first semiconductor layer 21 and the second semiconductor layer 22 because of the piezoelectric and spontaneous polarization effects induced there between. While the sensing device 100 operates to detect the target substance, the source and drain electrodes are connected to the 2DEG channel in an Ohmic contact type, and the sensing area 4 connects the 2DEG channel in Schottky contact type. If sensing area 4 is affected by the target substance, the target substance can change the charges on the sensing area 4. Therefore, the amount of the net surface charge changing on sensing area 4 can alter the electron density in the 2DEG channel. The detecting signal caused by the sensing area 4 can be amplified through the drain-source current. The drain-source current is varied along the change of the electron density in the 2DEG channel. In other words, the drain-source current is varied along the concentration of the target substance. Hence, the sensing device is very sensitive to the target substance and the detecting signal can be easily quantified, recorded, and transmitted.

[0032] The hetero-junction structure 2 can be made of a material which can enable the generation of a 2DEG layer. The first semiconductor layer 21 has at least one material different from the second semiconductor layer 22, and a wide bandgap between the first semiconductor layer 21 and the second semiconductor layer 22. Any material, such as III-V group materials, suitable for forming a heterojunction with 2DEG can be used to form the first semiconductor layer 21 and the second semiconductor layer 22. In order to change the 2DEG properties of heterojunction, the first semiconductor layer 21 and the second semiconductor layer 22 can be doped (for example with silicon) or un-doped. The first semiconductor layer 21 and the second semiconductor layer 22 can be made of gallium nitride (GaN), AlN, AlGaN, Al.sub.yIn.sub.zGa.sub.(1-z)N (0<y<1, 0<z<1), or etc. In one embodiment. the first semiconductor layer 21 can be made of GaN and the second semiconductor layer 22 can be made of aluminum gallium nitride (Al.sub.xGa.sub.(1-x)N), x=0.05.about.1. In other embodiments, the hetero-junction structure 2 can be made of material, such as AlGaN/InGaN/GaN, AlN/GaN, AlN/InGaN/GaN, AlGaAs/GaAs, AlGaAs/InGaAs, InAlAs/InGaAs, and InGaP/GaAs. The first semiconductor layer 21 has a thickness of a range 150.about.300 nm, for example 200 nm. The second semiconductor layer 22 has a thickness of a range 15.about.45 nm, for example 25.about.35 nm.

[0033] Referring to FIG. 1A and FIG. 1C, the sensing area 4 is separated from the electrodes 31, 32 by an insulating layer 8. The electrode 31 or 32 has a thickness of 2.about.5 .mu.m, for example, 3 .mu.m. The electrode 31 or 32 is made of one or more metallic materials, such as Au, Cu, Ti, Ni, Al, Pt, alloy thereof, or a combination thereof. The insulating layer 8 is made of one or more dielectric materials, such as, SiO.sub.2 or SiN.sub.x. The insulating layer 8 has a thickness of a range 500.about.1500 .ANG., for example 700.about.1000 .ANG..

[0034] Optionally, the sensing device 100 includes a functionalization layer 9 covering the sensing area 4. The functionalization layer 9 is used to enhance catalytic dissociation of the target substance. Specifically, the functionalization layer 9 can decompose one or more target substances in an environmental medium and facilitate the target substances diffusing rapidly to the sensing area 4. Hence the functionalization layer 9 is used to improve the detection limit and/or selectivity of the sensing device 100. The functionalization layer 9 is composed of one or more suitable materials for the target substance, for example, platinum is suitable for H.sub.2 detection. Other material of the functionalization layer 9 can be platinum, palladium, gold, nickel, iridium, or metal oxide like as SnO.sub.2, etc. The functionalization layer 9 has a thickness of a range 5.about.40 nm, for example 10.about.25 nm.

[0035] As shown in FIG. 1C, the substrate 1 has a trench 10 at the bottom side opposite to the side where the hetero-junction structure 2 resides. The trench 10 can have a smallest width which is larger than the biggest width of the sensing area 4 in a cross-sectional view. In more specific, the smallest width is the width of the top end 11 of the trench 10. The sensing area 4 is located right above the trench 10 and fully overlapped with the trench 10. In a top view or a bottom view (as shown in FIG. 1A or FIG. 1B), the trench 10 is located about the central part of the sensing device 100. The trench 10 and the sensing area 4 substantially have similar shapes (example, a rectangle, a square) in a top view or a bottom view.

[0036] As shown in FIG. 1C, the trench 10 penetrates the substrate 1. Hence, the depth of the trench 10 is substantially the same as the thickness of the substrate 1. Referring to the FIG. 1C, the substrate 1 has an inner surface 12 which surrounds the trench 10 and is inclined relative to the bottom surface of the hetero-junction structure 2. Therefore, the bottom end of the trench 10 has a width (area) which is larger than that of the top end 11 of the trench 10. The width (area) of the trench 10 gradually increases from the top end 11 to the bottom end in a cross-sectional view. In another embodiment, the inner surface 12 is perpendicular to the bottom surface of hetero-junction structure 2. The central part I of the sensing device 100 has a thickness which is smaller than that of the outer part II. The hetero-junction structure 2 located above the trench is not directly supported by the substrate 1. The substrate 1 can be any materials suitable for epitaxial growth, such as sapphire (Al.sub.2O.sub.3), silicon carbide (SiC), or silicon (Si). In one embodiment, the substrate 1 can be made of silicon (Si). The substrate 1 has a thickness of a range 200.about.400 .mu.m, for example 300 .mu.m.

[0037] As shown in FIG. 1D, in another embodiment, the trench 10 has a disc shape in the bottom view, and the sensing area 4 is fully overlapped with the trench 10 in the bottom view or the top view. The shape of the trench 10 in the bottom view is not limited to rectangle or disc, can have other shapes same or different from the sensing area 4, and the sensing area 4 is fully overlapped with the trench 10 in the bottom view or the top view.

[0038] As shown in FIGS. 1A.about.1C, the heater 5 is located in the trench 10 and disposed on the backside of the hetero-junction structure 2 opposite to location where the electrodes and sensing area 4 reside. In more detail, the heater 5 is formed on the top end 11 of the trench 10. As shown in FIG. 1A and FIG. 1B, two ends of the heater 5 are directly connected to the conducting pads 71, 72. The conducting pads 71 and 72 extend from the bottom side of the sensing device 100 to the top side of the sensing device 100 via two conducting through holes (not shown). The pair of the conducting pads 71, 72 can be electrically connected to an external power supply (not shown). The conducting pads 71, 72 are near the top side where the electrodes 31, 32 don't located, and the bottom side which is opposite to the top side of the sensing device 100 in the top view. The positions of the conducting pads 71, 72 are not limited to the top and bottom side of the sensing device 100 and can be arranged on any position of the sensing device 100 for compatible layout. The shape of the conducting pads 71, 72 includes, but is not limited to, a circle, a square, or a rectangle.

[0039] As shown in FIG. 1B and FIG. 1C, the heater 5 has a shape like as a meandering line, and the two ends are arranged in opposite sides. A plurality of the meandering portions 51 of the heater 5 is near the left side and the right side which are the sides the electrodes 31, 32 located thereon in the top view. The plurality of the meandering portions 51 has a right angle at the corner. In an embodiment (not shown), the plurality of the meandering portions 51 of the heater 5 is near the top side and the bottom side. The number of the meandering portions 51 can be varied according to the width, the material, the thickness, or other electrical parameters of the heater 5. In another embodiment, a portion of the meandering portions 51 of the heater 5 is near the left/right side and another portion of the meandering portions 51 of the heater 5 is near the top/bottom side. During operation, the electrical current passes through the heater 5 via the pair of the conducting pads 71, 72, and the heater 5 generates the heat to heat up the hetero-junction structure 2 and thermal couples to the sensing area 4. The more heat generated by the heater, the more operating current is needed to drive the heater.

[0040] The heater 5 is embedded in the sensing device 100 without bonding material or additional adhesive material. Moreover, the substrate 1 is open in a space where the heater 5 located, and the heat generated by the heater 5 can impose on the sensing area 4 without leaking to the substrate 1. The temperature of the sensing area 4 is close to the temperature of the heater 5. In other words, the heater 5 does not need to operate at high operating current, the sensing area 4 can reach the estimated temperature due to less heat leaking to the substrate. Hence, the consuming power of the heater 5 can be decreased. The shortest distance between the heater 5 and the sensing area 4 is less than 500 nm, for example 350 nm, 300 nm. The sensing device overall has a lower operating current due to the heater 5 with lower operating current. The operating current of the sensing device in accordance with the present disclosure is lower than 350 mA, for example lower than 200 mA, 150 mA, or 100 mA. The heater 5 is made of the material with the higher thermal conductivity, higher electrical resistivity, and lower coefficient of thermal expansion, such as Molybdenum (Mo), Polysilicon, silicon carbide, Ti, Ni, Platinum (Pt), Au, Al, Tungsten (W), SnO.sub.2, alloy thereof, or combinations thereof. The resistivity of the heater 5 depends on the material, the width, the length, the shape, and the thickness. The resistance of the heater 5 has a range of the 40.about.120 ohm, for example 50.about.100 ohm, 60 ohm, or 74 ohm. The thickness, width, and length of the heater 5 depend on the resistivity of the material of the heater. In an embodiment, the thickness of the heater has a range of the 1.about.10 .mu.m, for example 2.about.5 .mu.m, or 3 .mu.m.

[0041] A passivation layer 6 is located under and covers the heater 5. The passivation layer 6 can drive the heat generated from the heater 5 to move upward to the sensing area 4 via the hetero-junction structure 2. In other words, the passivation layer 6 can prevent the heat from going downward to the bottom side of the sensing device. As shown in FIG. 1C, the top end 11 of the trench 10 has a first portion 13 which is not covered by the heater 5, and a second portion 14 which is covered by the heater 5. The passivation layer 6 can fully cover the trench 10 (including the first portion 13 and the second portion 14). In other words, the passivation layer 6 is fully overlapped with the trench 10 in the bottom view or the top view. In one embodiment, the passivation layer 6 has a contour substantially similar to the heater 5. Hence, the first portion 13 is not fully covered by the passivation layer 6. In another embodiment, the passivation layer 6 covers the heater 5 and a portion of the first portion 13. Hence, a portion of the first portion 13 is exposed and another portion of the first portion 13 is covered by the passivation layer 6 in the bottom view. The covering area of the passivation layer 6 can vary as long as the heat generated from the heater 5 and moving upward to the sensing area 4. The passivation layer 6 is made of the dielectric material, such as, SiO.sub.2 or SiN.sub.x. In other words, the distance between the heater 5 and the sensing area 4 is decreased because the heater 5 is not separated from the hetero-junction structure 2 by the substrate 1. Further, the passivation layer 6 can drive the heat to move toward the sensing area 4. Hence, the heat wasted on the substrate would be decreased, and the power consumption of the heater 5 can be lower. Consequently, the overall power consumption of the sensing device can also be lower.

[0042] The shape of the heater 5 is not limited to a meandering line and can have other geometry for increasing the heat transferring to the sensing area 4 and the temperature uniformity of the sensing area 4.

[0043] FIGS. 2A.about.2E show other embodiments of heaters with different shapes. FIG. 2A shows a heater 5A which has a shape with a meandering line with two ends located on the same side. The heater 5A has a first portion 52A with a straight line and a second portion 53A with a "zigzag" meandering line. The second portion 53A has a plurality of the meandering portions 51A. The meandering portion 51A has a right angle at corner (the bending angle is substantially 90 degree). FIG. 2B shows another embodiment of the heater 5B which has a shape with a meandering line similar to the heater 5A. The heater 5B has a first portion 52B with a straight line and a second portion 53B with a "zigzag" meandering line. The second portion 53B has a plurality of the meandering portions 51B. The meandering portion 51B has an arc shape at the turning point to avoid the current accumulation at the turning point, and the reliability and cracking issue. FIG. 2C shows a heater 5C which has a shape with a double spiral. The heater 5C includes a first spiral portion 52C and a second spiral portion 53C. The first spiral portion 52C connects to the second spiral portion 53C via an interconnecting portion 54C. The first spiral portion 52C or the second spiral portion 53C has a plurality of the sectional lines 51C. The intersection of adjacent two sectional lines 51C has an angle .theta. which faces the geometric center. The angle .theta. is an obtuse angle to avoid the current accumulation at the intersection. In another embodiment, the intersection of adjacent two sectional lines 51C has an arc shape similar to FIG. 2B.

[0044] FIG. 2D shows a heater 5D which has a shape with circles. The heater 5D has an inner circle 52D and an outer circle 53D which are concentric with each other. The inner circle 52D and the outer circle 53D collectively form a ring. A gap 51D is formed between the interconnecting portions 54D which are used to connect the inner and outer circles 52D, 53D. The number of circles in not limited to the number exemplified herein and can include one or more than two circles with a gap between the interconnecting portions. And the plurality of the circles is concentric with each other. FIG. 2E shows a heater 5E with another shape. The heater 5E has an outer circle 53E and a disc 52E surrounded by the outer circle 53E and located around the geometric center. Two interconnecting portions 54E connect the outer circle 53E and the round shape 52E. The heater is not limited to shapes described above. The heater can have a shape with a combination of the aforementioned shapes. The heater 5 is made of a material with a higher electrical resistivity and a lower thermal conductivity, such as gold (Au), aluminum (Al), polysilicon, platinum (Pt), nickel (Ni), NiCr, molybdenum (Mo), tungsten (W), titanium (Ti), silicon carbide, graphite, or alloy thereof. The conducting pads 71, 72 are made of one or more metallic materials. The metallic material includes but not limited to Al, Cu, Au, Ag, Sn, Ti, Ni, and an alloy thereof.

[0045] FIG. 3 shows a sensing device 300 in accordance with an embodiment of the present disclosure. The sensing device 300 is similar to the sensing device 100 shown in FIG. 1C, and includes a substrate 1, a hetero-junction structure 2 formed on the substrate 1, electrodes 31, 32 located on the top side of hetero-junction structure 2, a sensing area 4 located on the top side of the hetero-junction structure 2. The hetero-junction structure 2 includes a first semiconductor layer 21 and a second semiconductor layer 22, and forms a two-dimensional electron gas (2DEG) channel 23 between the first semiconductor layer 21 and a second semiconductor layer 22. The substrate 1 has a trench 10. The trench 10 and the hetero-junction structure 2 are located on different sides of the substrate 1. The heater 5 is formed on the top end 11 of the trench 10. The passivation layer 6 covers the heater 5. The details width of the trench 10, the heater 5, and the passivation layer 6 can refer to aforementioned descriptions related to FIG. 1C. The substrate 1 can be partially removed or thinned down to form the trench 10 which does not pass through the substrate 10. The substrate 1 shown in FIG. 3 includes a thicker portion 15 and a thinner portion 16 surrounded by the thicker portion 15. The heater 5 is located in the trench 10 and formed on the thinner portion 16. The thinner portion 16, for example, has a thickness of a range 30.about.100 .mu.m. The thicker portion 15, for example, has a thickness of a range 200.about.400 .mu.m. In one embodiment, the thickness of the thinner portion 16 is of 45.about.55 .mu.m, the thickness of the thicker portion 15 is of 250.about.354 .mu.m. The area of substrate 1 right above the heater 5 has a smaller thickness, and the heat generated by the heater 5 can transfer to the sensing area 4 through a shorter passage. The distance between the heater 5 and the sensing area 4 is less than 100 .mu.m, for example 80 .mu.m, 70 .mu.m, or 57 .mu.m. Hence, the heater does not need to operate at higher operating current, and the sensing area 4 can reach the estimated temperature due to less heat leaking to the substrate. The power consumption of the heater 5 can be saved. Hence, the overall of the sensing device has a lower power consumption due to the heater is operated at a lower operating current level. The operating current of the sensing device 300 in accordance with the present disclosure is lower than 350 mA, for example, lower than 200 mA, 150 mA, or 100 mA. Optionally, the sensing device 300 includes a functionalization layer 9 covering the sensing area 4 to enhance the selectivity and the detection limit. The functionalization layer 9 is separated from the electrodes 31, 32 by an insulating layer 8. The details of the elements described herein can refer to aforementioned paragraphs directed to FIG. 1C.

[0046] FIG. 4 shows a sensing device 400 in accordance with an embodiment of the present disclosure. The sensing device 400 is similar to the sensing device 300 shown in FIG. 3, and includes a substrate 1, a hetero-junction structure 2 formed on the substrate 1, electrodes 31, 32 located on an top side of hetero-junction structure 2, a sensing area 4 located on the top side of the hetero-junction structure 2. The hetero-junction structure 2 includes a first semiconductor layer 21 and a second semiconductor layer 22, and can induce a 2DEG channel 23 between the first semiconductor layer 21 and a second semiconductor layer 22. The substrate 1 has a trench 10. The trench 10 and the hetero-junction structure 2 are located on different sides of the substrate 1. The heater 5 is formed on the top end 11 of the trench 10. The passivation layer 6 covers the heater 5. The details of the trench 10, the heater 5, and the passivation layer 6 can refer to aforementioned descriptions related to FIG. 1C. The second semiconductor layer 22 shown in FIG. 4 has a recess 20 on the side where the sensing area 4 resides. The second semiconductor layer 22 can be thinned down to form the recess 20 and the sensing area 4. In more detail, the second semiconductor layer 22 has a thicker portion 221 and a thinner portion 222 surrounded by the thicker portion 221. The sensing area 4 is located on the thinner portion 222 and lower than the topmost surface of the second semiconductor layer 22. The thinner portion 222 has a thickness of a range 5.about.15 nm. The thicker portion 221 has a thickness of a range 15.about.45 nm. In one embodiment, the thickness of the thinner portion 222 is of 6.about.10 nm, the thickness of the thicker portion 221 is of 25.about.35 nm.

[0047] The distance between the sensing area 4 and the 2DEG channel 23 is decreased due to the thin-down of the second semiconductor layer 22. The electric field between the sensing area 4 and the electrodes 31, 32 can be therefore caused a change by the target substances even with a lower concentration; for example, the concentration is less than 10 ppm. With a lower threshold of changing the electric field, the sensing area 4 becomes to be extremely sensitive to the ambient environment (gas). Hence, the sensing device can have a lower detection limit. Optionally, the sensing device 400 can include a functionalization layer 9 covering the sensing area 4 to further increase the selectivity and the detection limit. At least a portion of the functionalization layer 9 is surrounded by the second semiconductor layer 22. In one embodiment, the topmost surface 91 of the functionalization layer 9 is lower than the topmost surface of the thicker portion 221 of the second semiconductor layer 22. In another embodiment, the topmost surface 91 of the functionalization layer 9 is higher than or substantially coplanar to the topmost surface of the thicker portion 221 of the second semiconductor layer 22.

[0048] FIGS. 5A.about.5B show a sensing device 500 in accordance with an embodiment of the present disclosure. FIG. 5A shows the top view of the sensing device 500. FIG. 5B shows a partial cross-sectional view taken along line B-B' in FIG. 5A. The sensing device 500 is similar to the sensing device 300 shown in FIG. 3 and includes a substrate 1, a hetero-junction structure 2 formed on the substrate 1, electrodes 31, 32 located on the top side of hetero-junction structure 2, a sensing area 4 located on the top side of the hetero-junction structure 2. The hetero-junction structure 2 includes a first semiconductor layer 21 and a second semiconductor layer 22, and can induce a 2DEG channel 23 between the first semiconductor layer 21 and a second semiconductor layer 22. The substrate 1 has a trench 10. The trench 10 and the hetero-junction structure 2 are located on different sides of the substrate 1. The heater 5 is formed on the top end 11 of the trench 10. The passivation layer 6 covers the heater 5. The details of the substrate 1, the heater 5, and the passivation layer 6 can refer to aforementioned descriptions related to FIG. 1C, FIG. 3, or FIG. 4. The electrodes 31, 32 are interdigitated with each other for increasing the current spreading, called the interdigitated electrodes (IDE), in the top view. As shown in FIG. 5A, the electrode 31 has a first portion 311 close to a side 5001 of the sensing device 500, and a plurality of extending portions 312. The electrode 32 has a first portion 321 close to an opposite side 5002 of the sensing device 500, and a plurality of extending portions 322. The plurality of extending portions 312 of the electrode 31 extends from the first portion 311 toward but not contacting the first portion 321 of the electrode 32. A terminal of each of the plurality of the extending portions 312 is connected to the first portion 311. The plurality of extending portions 322 of the electrode 32 extends from the first portion 321 toward but not contacting the first portion 311 of the electrode 31. A terminal of each of the plurality of the extending portions 322 is connected to the first portion 321. The extending portions 312, 322 have a shape with a straight line substantially perpendicular to the first portions 311, 321 respectively, but not limited to. Referring to FIG. 5B, the extending portions 312 overlap at least a portion of the extending portions 322 from an outmost side 5003 to another outmost side 5004 opposite to the outmost side 5003 in the cross-sectional view. In one embodiment, the extending portions 312, 322 have curved shapes. The width of the extending portions 312 is different from, or same as that of the first portion 311. In one embodiment, the width of the extending portions 312 is smaller than the first portion 311.

[0049] The electrode 31 is separated from the electrode 32 by a non-zero distance. In more specific, an aisle 33 is formed between the electrodes 31, 32 and has a meandering path. The sensing area 4 is distributed in the aisle 33 and separated from the electrodes 31, 32 by the insulating layer 8. Optionally, the sensing device 500 includes a functionalization layer 9 covering on the sensing area 4 and located in the aisle 33. As shown in FIG. 5B, the topmost surface of the electrodes 31, 32 are higher than that of the functionalization layer 9. In another embodiment, the second semiconductor layer 22 has a recess and includes a thinner portion and a thicker portion similar to the sensing device 400 shown in FIG. 4. Hence, the distance between the sensing area 4 and the two-dimensional electron gas (2DEG) channel 23 can be decreased for improving the detection limit.

[0050] FIGS. 6A.about.6F show steps of manufacturing a sensing device in accordance with an embodiment of the present disclosure. As shown in FIG. 6A, a substrate 1 is provided. The hetero-junction structure 2 including a first semiconductor layer 21 and a second semiconductor layer 22 is epitaxially grown on the substrate 1 by deposition method, such us Metal Organic Chemical Deposition (MOCVD) or molecular beam exitaxy (MBE). As shown in FIG. 6B, the electrodes 31, 32 are formed on the hetero-junction structure 2. Next, as shown in FIG. 6C, the insulating layer 8 is formed on the electrodes 31, 32, and the hetero-junction structure 2. The insulating layer 8 exposes a portion of the top surface of the electrodes 31, 32 to electrically connect to the driving power, and exposes a portion of the top surface of the hetero-junction structure 2 to define the sensing area 4. Optionally, the second semiconductor layer 22 of the hetero-junction structure 2 can be thinned down by etching process to form a recess. Optionally, the functionalization layer 9 is then deposited on the sensing area 4. Then, as shown in FIG. 6D, the structure shown in FIG. 6C is reversed and the substrate 1 is removed or thinned down by etching to form the trench 10. Next, referring to FIG. 6E, the heater 5 with a pattern is formed on the top end 11 of the trench 10. Then, referring to FIG. 6F, the passivation layer 6 is disposed to cover the heater 5. At last, the structure is faced up to form a sensing device.

[0051] FIGS. 7A.about.7B show a sensing device 600 in accordance with an embodiment of the present disclosure. FIG. 7A shows the top view of the sensing device 600. FIG. 7B shows a partial cross-sectional view taken along line C-C' in FIG. 7A. The sensing device 600 includes a substrate 1, a hetero-junction structure 2 formed on the substrate 1, electrodes 31, 32 located on the top side of hetero-junction structure 2, a sensing area 4 located on the top side of the hetero-junction structure 2. The hetero-junction structure 2 includes a first semiconductor layer 21 and a second semiconductor layer 22, and can induce a 2DEG channel 23 between the first semiconductor layer 21 and a second semiconductor layer 22. The heater 5 and the sensing area 4 are located on the top side of hetero-junction structure 2. The heater 5 and the electrodes 31, 32 are separated and isolated from each other by the insulating layer 8. The heater 5 has two terminals 51, 52 connected to the conducting pads 71, 72 which are located on the top side of the hetero-junction structure 2. The passivation layer 6 is sandwiched by the heater 5 and the hetero-junction structure 2 to electrically isolate the heater 5 from the hetero-junction structure 2. Referring to FIG. 7A, the heater 5, the sensing area 4, the electrodes 31, 32, and the conducting pads 71, 72 are on the same side and on the top side of the hetero-junction structure 2.

[0052] The electrodes 31, 32 have a shape similar to the sensing device 500 shown in FIG. 5A. The electrode 31 has a first portion 311 close to a side 6001 of the sensing device 600 and a plurality of extending portions 312. The electrode 32 has a first portion 321 close to an opposite side 6002 of the sensing device 600 and a plurality of extending portions 322. The plurality of extending portions 312 of the electrode 31 extends from the first portion 311 toward but not contacting the first portion 321 of the electrode 32. A terminal of each of the plurality of the extending portions 312 is connected to first portion 311. The plurality of extending portions 322 of the electrode 32 extends from the first portion 321 toward but not contacting the first portion 311 of the electrode 31. A terminal of each of the plurality of the extending portions 322 is connected to the first portion 321. In other words, the electrodes 31, 32 are interdigitated with each other in the top view. One of plurality of the extending portions 312 and one of plurality of the extending portions 322 are aligned in a line and separated by a non-zero distance in the top view. The plurality of the extending portions 312 does not overlap with the plurality of the extending portions 322 from the left side 6003 to the right side 6004 in the top view. The electrode 31 and the electrode 32 are mirror symmetric with each other. The extending portion 312 has a straight line perpendicular to the first portion 311. The extending portion 322 has a straight line perpendicular to the first portion 321. In another embodiment, the extending portion 312 (322) is not perpendicular to the first portion 311 (321).

[0053] As shown in FIG. 7A, the heater 5 and the sensing area 4 are located in the area which is between the electrode 31 and the electrode 32. The heater 5 has a first portion 54, a second portion 55, a third portion 56, and two terminals 51, 52. The third portion 56 connects the first portion 54 and the second portion 55. The terminal 51 is located at an end of the first portion 54 which is opposite to the third portion 56. The terminal 52 is located at an end of the first portion 55 which is opposite to the third portion 56. The heater 5 is connected to the conducting pads 71, 72 via the terminals 51, 52. The electrode 31 has an inner side 313 facing the electrode 32, and the electrode 32 has an inner side 323 facing the electrode 31. The first portion 54 of the heater 5 and the inner side 313 have similar profiles, and the second portion 55 of the heater 5 and the inner side 323 have similar profiles. In more specific, the heater 5 forms in a shape similar to a loop with a small opening 53 sandwiched by two terminals 51, 52. The heater 5 has a shape formed along the inner sides 313, 323 of the electrodes 31, 32. The area within the loop formed by the heater 5 is the sensing area 4. The sensing area 4 is surrounded by the heater 5. From the top view as shown in FIG. 7A, the heater 5 is adjacent to the sensing area 4 by a distance close to zero.

[0054] Optionally, the functionalization layer 9 is formed on the sensing area 4 to improve the detection limit and the selectivity, such as the sensing device 700 shown in FIGS. 8A.about.8B. FIGS. 8A.about.8B show a sensing device 700 in accordance with an embodiment of the present disclosure. FIG. 8A shows the top view of the sensing device 700. FIG. 8B shows a partial cross-sectional view taken along line D-D' in FIG. 8A. The sensing device 700 is similar to the sensing device 600 shown in FIGS. 7A.about.7B. The sensing device 700 includes a substrate 1, a hetero-junction structure 2 formed on the substrate 1, electrodes 31, 32 located on the top side of hetero-junction structure 2, a sensing area 4 located on the top side of the hetero-junction structure 2. The heater 5 and the sensing area 4 are located on the top side of hetero-junction structure 2. The heater 5 connects to the conducting pads 71, 72. The heater 5 and the conducting pads 71, 72 are located on the top side of the hetero-junction structure 2. The passivation layer 6 is sandwiched by the heater 5 and the hetero-junction structure 2 to electrically isolate the heater 5 from the hetero-junction structure 2. The functionalization layer 9 is formed on the sensing area 4. The functionalization layer 9 and the heater 5 (or the passivation layer 6) is separated by the insulating layer 8. The distance between the heater 5 and the sensing area 4 is less than 80 .mu.m in the top view, for example 70 .mu.m, 50 .mu.m, or 20 .mu.m. In the top view, as shown in FIG. 8A, the area surrounded by the heater 5 is slightly larger than the area of the functionalization layer 9 (or the sensing area 4). As shown in FIG. 8B, the thickness of the functionalization layer 9 is thicker than that of the passivation layer 6. In another embodiment, the thickness of the functionalization layer 9 is thinner than that of the passivation 6. Optionally, there is no insulating layer 8 formed between the functionalization layer 9 and the heater 5 (or the passivation layer 6). Hence, the distance between the functionalization layer 9 and the heater is close to zero in the top view.

[0055] The substrate 1 of the sensing device 600, 700 can have a trench 10 as shown in FIG. 9. FIG. 9 shows a partial cross-sectional view of the sensing device 800 in accordance with an embodiment of the present disclosure. The electrodes 31, 32, heater 5, and the hetero-junction structure 2 can refer to the descriptions directed to FIGS. 7A.about.7B and FIGS. 8A.about.8B. The substrate 1 of the sensing device 800 is similar to that of the sensing device 100, 200, 300, 400, or 500. The substrate 1 is thinned down to form the trench 10 which is right under the sensing area 4. The substrate 1 has a thinner portion 16 and a thicker portion 15. The thinner portion 16 is surrounded by the thicker portion 15. In another embodiment, the substrate 1 is partially removed under the sensing area 4 to form the trench 10. The substrate 1 which is thinned down or partially removed can avoid the heat generated by the heater 5 from leaking to the substrate 1. The heater 5 can cause the power consumption decreased, and the sensing area 4 to reach the estimated temperature easily. Hence, the operating current of the sensing device can be lower due to the operating current of the heater is lower, for example less than 350 mA, 200 mA, or 100 mA.

[0056] In order to enhance the detection limit, the second semiconductor layer 22 of the hetero-junction structure 2 can be thinned down to form a recess, as shown in FIG. 4. FIG. 10 shows a partial cross-sectional view of the sensing device 900 in accordance with an embodiment of the present disclosure. The details of the electrodes 31, 32, the heater 5, and the substrate 1 can refer to the aforementioned descriptions related to FIGS. 7A.about.7B, FIGS. 8A.about.8B, or FIG. 9. The second semiconductor layer 22 shown in FIG. 10 includes a recess where the sensing area 4 resides. The second semiconductor layer 22 has a thicker portion 221 and a thinner portion 222 surrounded by the thicker portion 221. The sensing area 4 is lower than the topmost surface of the second semiconductor layer 22 and surrounded by the second semiconductor layer 22.

[0057] FIG. 11 shows a partial cross-sectional view of the sensing device 1000 in accordance with an embodiment of the present disclosure. The substrate 1, and the hetero-junction structure 2 can refer to aforementioned descriptions related to the sensing device 600, 700, 800, or 900. The electrodes 31, 32, the heater 5, and the sensing area 4 of the sensing device 1000 are located on the top side of the hetero-junction structure 2 and concentric with each other in the top view. Optionally, the functionalization layer 9 is formed on the sensing area 4 to enhance the selectivity and the detection limit. Referring to FIG. 11, the electrodes 31, 32 collectively formed on a first virtual circle C1. The heater 5 is formed on a second virtual circle C2 and the fourth virtual circle C4. The sensing area 4 is formed a third virtual circle C3 and a central area C5. C1, C2, C3, C4, and C5 are arranged in sequence from outer to inner and concentric with each other in the top view. The electrode 31 has a curved portion 311 close to a side 10001 of the sensing device 1000. The electrode 32 has a curved portion 312 close to an opposite side 10002 of the sensing device 1000. The curved portions 311, 312 are separated from each other and located on the first virtual circle C1. The heater 5 has a first curved portion 51, a second curved portion 52, a third portion 53, and the interconnecting portions 54 connecting the first curved portion 51, the second curved portion 52, and the third portion 53. The first portion 51 and the second portion 52 are separated from each other and located on the second virtual circle C2. The third portion 53 is located on the fourth virtual circle C4. The sensing area 4 includes a first portion 41 and a second portion 42 separated from the first portion 41. The sensing area 4 is surrounded by the heater 5. The first portion 41 of the sensing area 4 is sandwiched by the first curved portion 51, the second curved portion 52, and the third portion 53, and located on the third virtual circle C3. The second portion 42 of the sensing area 4 has a circular shape located on the central area C5 and is surrounded by fourth virtual circle C4 where the third portion 53 of the heater 5 resides. The electrodes 31, 32, heater 5, and the sensing area 4 are arranged to decrease distances between the heater 5 and the sensing area 4 for enhancing the current spreading. The arrangement of the electrodes 31, 32, heater 5, and the sensing area 4 is not limited to circular and alternating configuration as exemplified herein.

[0058] FIGS. 12A.about.12C show steps of manufacturing a sensing device 600 in accordance with an embodiment of the present disclosure. As shown in FIG. 12A, a substrate 1 is provided and the hetero-junction structure 2 including a first semiconductor layer 21 and a second semiconductor layer 22 is epitaxially grown on the substrate 1 by deposition method, such us Metal Organic Chemical Deposition (MOCVD) or molecular beam exitaxy (MBE). As shown in FIG. 12B, the electrodes 31, 32 are formed on the hetero-junction structure 2. Next, as shown in FIG. 12C, the insulating layer 8 is formed on the electrodes 31, 32, and the hetero-junction structure 2. The passivation layer 6 and the heater 5 are deposited on the hetero-junction with the specific pattern to define the region of the sensing area 4. Optionally, the second semiconductor layer 22 of the hetero-junction structure 2 is thinned down to form a recess before forming the passivation layer 6 and the heater 5. Then, optionally, the functionalization layer 9 is deposited on the sensing area 4. Next, as shown in FIG. 12D, optionally, the substrate 1 is removed or thinned down by etching to form the trench 10.

[0059] The sensor resistance (Rs) is the resistance between the two electrodes, defined as Rs=(V.sub.C.times.R.sub.L)/V.sub.out-R.sub.L. V.sub.C is the voltage applied to the two electrodes. R.sub.L is the loading resistor connected in series with the output terminal via one of electrodes. The V.sub.out is the voltage across the load resistor R.sub.L. The sensitivity .beta. of the sensing device is defined as a ratio of a target sensor resistance to a reference sensor resistance (change ratio of sensor resistance). For example, the reference sensor resistance Rs (30 ppm) of the sensing device is set to be the resistance value measured under the target substance with 30 ppm concentration. The target sensor resistance Rs (100 ppm) of the sensing device is set to be the resistance value measured under the target substances with 100 ppm concentration. Then, the sensitivity of the sensing device can be presented as .beta.=Rs (100 ppm)/Rs (30 ppm). V.sub.out varies in accordance to the output current of the sensing device; hence, the sensitivity also can be presented by the variation of the output current. When the sensitivity .beta. is higher, the reading circuit connected to the output of the sensing device is easier to read out without complicated amplification. The detection limit is defined as the minimum concentration of the target substance to trigger the sensor. The detection limit of the embodiment aforementioned is less than 10 ppm, for example less than 7 ppm, 5 ppm, 1 ppm, or 0.5 ppm.

[0060] It will be apparent to those having ordinary skill in the art that various modifications and variations can be made to the devices in accordance with the present disclosure without departing from the scope or spirit of the disclosure. In view of the foregoing, it is intended that the present disclosure covers modifications and variations of this disclosure provided they fall within the scope of the following claims and their equivalents.

Claims

1. A sensing device, comprising: a semiconductor structure comprising a top side, a bottom side opposite to the top side, and a sensing area arranged on the top side; a substrate arranged under the bottom side; a first electrode and a second electrode arranged on the top side in a configuration to expose the sensing area; and a heater disposed on the semiconductor structure and separated from the sensing area by a distance of less than 100 .mu.m.

2. The sensing device according to claim 1, wherein the substrate has a trench which is overlapped with the sensing area in a top view.

3. The sensing device according to claim 2, wherein the substrate has a thinner portion and a thicker portion surrounding the thinner portion.

4. The sensing device according to claim 2, wherein the trench has a top end where the heater is located, and an open end opposite to the top end.

5. The sensing device according to claim 1, further comprising a passivation layer covering the heater.

6. The sensing device according to claim 1, further comprising a passivation layer sandwiched by the heater and the semiconductor structure.

7. The sensing device according to claim 1, further comprising a passivation layer formed in a loop configuration to enclose the sensing area.

8. The sensing device according to claim 1, wherein the first electrode and the second electrode are interdigitated with each other in a top view.

9. The sensing device according to claim 1, wherein the semiconductor structure has a recess formed on the top side where the sensing area is located.

10. The sensing device according to claim 1, wherein the heater comprises a meandering line.

11. A sensing device, comprising: a semiconductor structure comprising a top side, a bottom side opposite to the top side, and a sensing area arranged on the top side; a substrate arranged under the bottom side; an electrode disposed on the top side to expose the sensing area; and a heater disposed on the semiconductor structure; wherein the sensing device has an operating current less than 350 mA.

12. The sensing device according to claim 11, wherein the heater is separated from the sensing area by a distance of less than 100 .mu.m.

13. The sensing device according to claim 11, wherein the heater is located on the top side.

14. The sensing device according to claim 11, further comprising a passivation layer sandwiched by the heater and the semiconductor structure.

15. The sensing device according to claim 11, further comprising a passivation layer formed in a loop configuration to enclose the sensing area.

16. A sensing device, comprising: a semiconductor structure comprising a top side, a bottom side opposite to the top side, and a sensing area arranged on the top side; a substrate arranged under the bottom side; an electrode disposed on the top side in a configuration to expose the sensing area; and a heater disposed on the semiconductor structure, wherein the sensing device has a detection limit less than 10 ppm.

17. The sensing device according to claim 16, wherein the heater is separated from the sensing area by a distance of less than 100 .mu.m.

18. The sensing device according to claim 16, wherein the heater is located on the top side.

19. The sensing device according to claim 16, further comprising a passivation layer sandwiched by the heater and the semiconductor structure.

20. The sensing device according to claim 16, wherein the substrate has a trench which is overlapped with the sensing area in a top view.